Modeling of MHD equilibria and current profile evolution during the ERS mode in TFTR

Description

TFTR experiments on the enhanced reversed shear (ERS) mode have demonstrated particle and ion thermal diffusivities in the region of negative shear which are equal to or less than the neoclassical values. Similar enhancements have been observed in reversed central shear discharges in the shaped DIII-D geometry. These results, if sustained over times long compared with current diffusion times, offer the opportunity of an improved reactor. We are modeling the evolution of the TFTR ERS mode using Corsica, a predictive 1-1/2 D equilibrium code. Similar modeling is being done for DIII-D; the common goal is to better understand the physics ...
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Description

TFTR experiments on the enhanced reversed shear (ERS) mode have demonstrated particle and ion thermal diffusivities in the region of negative shear which are equal to or less than the neoclassical values. Similar enhancements have been observed in reversed central shear discharges in the shaped DIII-D geometry. These results, if sustained over times long compared with current diffusion times, offer the opportunity of an improved reactor. We are modeling the evolution of the TFTR ERS mode using Corsica, a predictive 1-1/2 D equilibrium code. Similar modeling is being done for DIII-D; the common goal is to better understand the physics of the discharges in order to predict performance and eventually to provide a capability of real-time control of the profiles. Here we describe a first step in applying Corsica to the TFTR discharges. We first examine the equilibria generated in TRANSP, using the output pressure and safety factor, q, (or the parallel current) profiles to regenerate the magnetic equilibria. Two TRANSP options are used: (1) a minor radius- like coordinate is used as a flux surface label, or (2) toroidal flux is used to label the surfaces. Our equilibria agree much better with option (1) than (2). However, we still find incompatibilities among the profiles, viz. fixing the q and p profiles yields a current profile somewhat different from TRANSP. The second step in the analysis presented here is to compare the time evolution of the q and current profiles with experiment. The calculation is initialized at a time before the neutral beams are ramped up; the evolution is followed through the reverse central shear period, using as inputs the pressure and current drive results from the TRANSP analysis. The calculation is thus an evaluation of the magnetic field diffusion due to neoclassical resistivity; the result is compared with the experimental results. The calculated q profiles agree reasonably well with experiment. 8 refs., 8 figs.